Optical communications module and method for producing the module

ABSTRACT

An optical communications module in which electrical crosstalk is reduced. The term “optical communications module” represents a surface-mounting-type optical tranceiver, transmitter, or receiver module. The optical communications module has a following structure. (a) An Si substrate carries at least one signal-transmitting section comprising an LD, at least one signal-receiving section comprising a PD, or both together with other components. (b) An insulating substrate is bonded to the back face of the Si substrate. (c) A separating groove separates the Si substrate along the or each boundary line between the sections in order to prevent an AC current flowing through the Si substrate. To attain this object, the separating groove is provided from the top surface of the Si substrate to some midpoint of the insulating substrate. This structure reduces the electrical crosstalk between the signal-transmitting section and the signal-receiving section, between the signal-transmitting sections, and between the signal-receiving sections.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical communications module foruse in optical communications which is provided with at least one laserdiode, at least one photodiode, or both for transmitting and receivingoptical signals, and particularly to an optical communications module inwhich electrical crosstalk between the signal-transmitting section andthe signal-receiving section, between the signal-transmitting sections,and between the signal-receiving sections is reduced. The presentinvention also relates to a method for producing the opticalcommunications module.

2. Description of the Background Art

In the optical communications system using light for transmittinginformation, the optical communications modules described below areknown for transmitting and receiving optical signals propagating over anoptical transmission line such as an optical fiber transmission line.

-   -   (a) Ryuta Takahashi et al. have proposed the following optical        module in the report entitled “Packaging of optical        semiconductor chips for SFF (small form factor) optical        transceiver” included in the proceedings of the 1999 Electronics        Society Conference of IEICE (The Institute of Electronics,        Information and Communication Engineers of Japan) (page 133,        number C-3-28). The optical module incorporates an Si substrate        provided with two V-shaped grooves and metallized patterns        (electrode patterns). A laser diode (hereinafter referred to as        an LD) for signal transmission and a photodiode (hereinafter        referred to as a PD) for signal reception are placed on the        metallized patterns. Optical fibers are securely held in the        V-shaped grooves to be butt-connected with the LD and PD for        enabling the interchange of the signal light.

The module is illustrated in FIGS. 9 and 10. FIG. 9 is a plan view andFIG. 10 a longitudinal cross section. An insulating layer 3 (SiO₂) isformed on the rear half of a rectangular Si substrate 2. An LD 4 forsignal transmission and a PD 5 for signal reception are mounted on theinsulating layer 3. Two parallel V-shaped grooves 7 and 8 are formed onthe front half 6 of the Si substrate 2. The V-shaped grooves are formedwith high precision by the anisotropic etching of an Si single crystal.Optical fibers 9 and 10 are fitted in the V-shaped grooves 7 and 8 andfixed there. The end of the optical fiber 9 faces the LD 4 and that ofthe optical fiber 10 faces the PD 5. As shown in FIG. 10, alight-emitting layer 27 of the LD 4 is positioned at the same level asthat of the center of the optical fiber 9. Light emitted from the sideface of the LD 4 propagates over the optical fiber 9 to the outside.Conversely, light from the outside optical fiber propagates over theoptical fiber 10 to reach the side face of the PD 5. Therefore, the PD 5must be a side-illuminated type. Front- and rear-illuminated typescannot be used.

The LD 4 and PD 5 are placed with left-right symmetry with respect tothe centerline of the Si substrate 2. Consequently, the optical fibersare parallel to each other, and the LD 4 and PD 5 are parallel to eachother. This configuration can reduce the space needed. Because the LD 4and PD 5 are mounted on the same Si substrate 2 in parallel, the modulecan reduce the parts prices, mounting cost, and size.

The Si substrate 2 has a rectangular shape with a length of 5 mm and awidth of 2.5 mm. Actually, the optical fibers, LD 4, PD 5, and othercomponents are covered with a transparent resin. Furthermore, the entireunit is encapsulated with a shape-holding resin to form a module of aplastic-molded package type.

-   -   (b) The published Japanese patent application Tokukaihei        11-68705 entitled “bidirectional optical WDM (wavelength        division multiplexing) tranceiver module” has proposed a        tranceiver module. In the tranceiver module, an SiO₂ insulating        layer is formed on an Si substrate. A Y-shaped optical waveguide        made of GeO₂-doped SiO₂ enclosed by SiO₂ is formed on the        insulating layer. A PD is placed at the position corresponding        to the bottom end of the letter Y An optical fiber is connected        to the position corresponding to the left upper end of the        letter Y. An LD is placed at the position corresponding to the        right upper end of the letter Y. A WDM filter that selectively        reflects 1.3-μm light and selectively transmits 1.55-μm light is        placed at the junction point of the letter Y. The 1.3-μm light        for signal transmiission emitted from the LD propagates over the        optical waveguide, is reflected by the filter, enters the        optical fiber, and transmits to the outside.

Conversely, light having propagated over a optical fiber from theoutside enters the optical waveguide, passes through the filter, andenters the PD to give signals. The PD, also, is a side-illuminated type.Front- and rear-illuminated types cannot be used. In this tranceivermodule, the LD and PD are placed with front and rear symmetry, not withleft-right symmetry.

Both tranceiver modules explained in (a) and (b) above have a structurein which an LD and a PD are mounted on an Si substrate, and they areconnected to the outside optical fiber through an optical transmissionmedium such as an optical waveguide or fiber. The propagation directionof light, the chip faces, and the substrate surface are parallel to oneanother. Consequently, light propagates two-dimensionally withoutrequiring a wide space. As a result, a tiny, low-cost optical tranceivermodule can be produced.

However, the tiny, low-cost optical tranceiver module newly makes aproblem of electrical crosstalk caused by the intrusion of noise fromthe signal-transmitting section to the signal-receiving section. Thereare two causes for the crosstalk. FIG. 11 is a lateral cross section ata plane including the LD and PD in the optical tranceiver module shownin FIG. 9 and 10. The causes of the crosstalk are explained as followsby using Fig.11.

-   -   (1) The insulating layer 3 (SiO₂ layer) sandwiched between the        metallized patterns 20 and 24 and the Si substrate 2 is a        dielectric material and has capacitances C2 and C3.    -   (2) The Si substrate 2 is a semiconductor substrate having        finite resistances R4, R5, R6, and R7.

Therefore, the LD 4 and the PD 5 are connected to each other through anAC circuit composed of the resistances R4, R5, R6, and R7 in the Sisubstrate 2 and the equivalent capacitances C2 and C3 in the insulatinglayer 3. Because the insulating layer 3 is thin, the capacitances C2 andC3 are high. Because the frequency is high, the reactances 1/jωC2 and1/jωC3 are low. The Si substrate 2 is made of an n- or p-type Si singlecrystal having low resistivity for a semi-conductor. Consequently, thevalue of R4+R5+R6+R7 is considerably low. As a result, the totalimpedance Z=1/jωC2+1/jωC3+R4+R5+R6+R7 is considerably low. This lowimpedance allows intense electrical signals to be fed to the LD to leakinto the PD and mix with received signals. Thus, electrical crosstalk iscaused between the LD and PD through the Si substrate.

-   -   (c) Sonomi Ishii et al. have proposed an idea to reduce the        crosstalk between the signal-transmitting section and the        signal-receiving section in the report entitled “Crosstalk        analysis of MT-RJ optical subassembly” included in the        proceddings of the 2000 Electronics Society Conference of IEICE        (page 352, number SC-3-7). The idea employs a shield plate        inserted into a groove provided at a location between the        signal-transmitting section and the signal-receiving section on        an Si substrate. The groove has a depth less than the thickness        of the Si substrate. The distance between the optical axes of        the two sections is 750 μm. It can be said that the shield plate        is provided to prevent crosstalk caused by electromagnetic waves        rather than by electric current. To block electromagnetic waves        intruding from the LD to the PD, the shield plate is        substantially high from the surface of the Si substrate.        However, because the impedance Z=1/ωC2+1/ωC3+R4+R5+R6+R7 is low,        the electrical crosstalk caused by an AC current flowing through        the Si substrate remains high.    -   (d) The published Japanese patent application Tokukai        2002-170984 (filed as Tokugan 2000-367925) entitled “Optical        communications unit” has proposed a structure to reduce        electromagnetic crosstalk. In the proposed structure, a series        of through holes are provided somewhere between the LD and PD on        the Si substrate. A metallic shield plate having a comb-shaped        foot is inserted into the holes. The foot is soldered to a        metallic base plate attached to the back face of the Si        substrate. The LD and PD are isolated from each other with a        metallic box incorporating the metallic shield plate as a        separator. The structure is intended to completely suppress        crosstalk caused by electromagnetic waves. Electric signals for        the LD emit substantially intense electromagnetic waves because        they are composed of high-speed pulses. The electromagnetic        coupling between the LD and PD placed in proximity to each other        allows the electro-magnetic waves to intrude into the PD as        noises. To prevent the intrusion, the prior art (d) has devised        a structure in which the electromagnetic waves are contained in        the metallic box that separately encases the LD and PD sections.

There are three types of crosstalk between the signal-transmittingsection (LD section) and the signal-receiving section (PD section):optical crosstalk, electromagnetic crosstalk, and electrical crosstalk.Because the signals have high frequency, electromagnetic crosstalk isnoticeable. However, attention must be paid to the presence ofelectrical crosstalk. In the situation under consideration, the Sisubstrate 2 is a good conductor rather than a semiconductor. Theinsulating layer 3 is thin and allows AC current to flow. As a result,electrical crosstalk becomes significant. The prior arts (a) and (b)described above have no effective measure against the problem of thecrosstalk between the signal-transmitting and -receiving sections. Theprior arts (c) and (d) merely describe measures against electromagneticcrosstalk. It cannot be said that sufficient attention is paid to theelectrical crosstalk in the prior arts cited above.

The fact that the Si substrate is a good conductor in this case may havebeen neglected in the above-described prior arts. If the substrate isproduced by using an insulating material such as a ceramic material, theproblem of the electrical crosstalk may be solved. However, a ceramicsubstrate is devoid of the advantages the Si substrate innately has. TheSi substrate is advantageous in that photolithographic techniques formwith extremely high precision the V-shaped grooves, the positioningmarks for the LD and PD, and the various patterns, and substrates can bemass-produced using a large wafer. Now, Si single crystals are producedby the most advanced single-crystal production technology. Large Siwafers (300 mm in diameter) are available at low prices. The productiontechnology of Si wafers has been highly sophisticated to such an extentthat the formation of the V-shaped grooves and marking can be easilyperformed with high precision. The Si substrate has attractive featuresthat cannot be obtained with a ceramic substrate.

SUMMARY OF THE INVENTION

As described above, it is urgently required to eliminate the electricalcrosstalk through a substrate without losing the advantageous featuresthat a semiconductor substrate, such as an Si substrate, innately has.An object of the present invention is to offer a structure that canreduce the electrical crosstalk in an optical communications modulehaving at least one signal-transmitting section and at least onesignal-receiving section, a plurality of signal-transmitting sections,or a plurality of signal-receiving sections on the Si substrate.

According to the present invention, the foregoing object is attained byan optical communications module that has the following structure:

-   -   (a) An Si substrate carries a pair of a signal-transmitting        section and a signal-receiving section, a plurality of        signal-transmitting sections, a plurality of signal-receiving        sections, or a plurality of pairs of a signal-transmitting        section and a signal-receiving section.    -   (b) An insulating substrate is bonded to the back face of the Si        substrate.    -   (c) A separating groove is provided to separate the Si substrate        along the or each boundary line between the foregoing sections.        For this purpose, the separating groove is provided from the top        surface of the Si substrate to some midpoint of the insulating        substrate.

The optical communications module may further have at least one opticalwaveguide formed on the Si substrate. In place of this structure, theoptical communications module may further have the following structure:

-   -   (a) At least one V-shaped groove is formed on the Si substrate.    -   (b) An optical fiber is fixed in the or each V-shaped groove.

The optical communications module may have a structure in which the oreach signal-transmitting section comprises a laser diode.

The optical communications module may have a structure in which the oreach signal-receiving section comprises a photodiode.

Because, the foregoing separating groove separates the Si substrate, thetotal summation of the above-described series resistances formed throughthe Si substrate becomes nearly infinity, as described below. When theseries resistance in the separating groove is denoted by R8, the amountof R8 is nearly infinity and can be expressed as R8=∞. Consequently, thetotal summation of the resistances is expressed as R4+R5+R8+R6+R7=∞. Onthe other hand, the separating groove also produces an additionalcapacitance C9. Therefore, the total impedance is expressed asZ=1/ωC2+1/jωC3+R4+R5+1/jωC9+R6+R7. However, when the width Wof theseparating groove is widened, the capacitance C9 can be reducedconsiderably. As a result, the total impedance can be increased.

According to the present invention, the separating groove separates theSi substrate so as to isolate the signal-transmitting section and thesignal-receiving section from each other. When the Si substrate carriesa plurality of signal-transmitting sections, the separating grooveisolates the signal-transmitting sections from each other. When the Sisubstrate carries a plurality of signal-receiving sections, theseparating groove isolates the signal-receiving sections from eachother.

When the Si substrate alone is used, the separating groove take the Sisubstrate to pieces. To maintain the integral structure of the Sisubstrate, an insulting substrate is bonded to the back face of the Sisubstrate before the separating groove is formed. Even when theseparating groove penetrating into the insulating substrate to a certainextent is formed from the top surface of the Si substrate, theinsulating substrate can maintain the original position of the separatedSi substrate.

If the Si substrate is severed without using the insulating substrate,the optical fiber, LD, and PD cannot be mounted with highprecision. Ifhigh precision cannot be attained, there is no point in using thesingle-crystalline Si substrate. According to the present invention,after the insulating substrate is bonded to the back face of the Sisubstrate, the optical fiber, LD, and PD are mounted on the Si substratewith high precision. Subsequently, the separating groove penetratinginto the insulating substrate to a certain extent is formed from the topsurface of the Si substrate.

When the thickness of the Si substrate is denoted by s, the depth of theseparating groove by g, and the thickness of the insulating substrate byq, the depth of the separating groove ghas the following relationship:s<g<s+q   (1).Although the Si substrate is electrically separated completely by theseparating groove (R8=∞), the separated portions are mechanicallyconnected by the insulating substrate.

The Si substrate used for a surface-mounting-type optical communicationsmodule usually has a thickness of 0.5 to 1.5 mm or so. It issubstantially thick, because it alone must support optical devices andoptical fibers. The Si substrate for the present invention, also, mayhave a thickness of 0.5 to 1.5 mm. According to the present invention,however, the thickness may be reduced, because the Si substrate isreinforced by the insulating substrate. For example, the thickness maybe reduced to the range of 0.06 to 0.5 mm.

As described above, in the present invention, an insulating substrate isbonded to the back face of the Si substrate so that the Si substrate canbe separated with a separating groove. Without the insulating substrate,the Si substrate goes to pieces by the separating groove. The insulatingsubstrate is bonded to the Si substrate before the Si substrate isseparated. If the Si substrate is separated before the bonding of theinsulating substrate and is mounted onto the insulating substrate insuch a manner as to fit together the pieces of a jigsaw puzzle, themounting cannot be performed with high precision. If high precisioncannot be attained, there is no point in using the Si substrate. Thetypes of insulating substrates include a ceramic substrate and a plasticsubstrate.

According to the present invention, various components such as an LD,PD, optical fiber, and optical waveguide are mounted on the Si substratereinforced by the insulating substrate. Before or after the mounting ofthe foregoing components, the Si substrate is separated to isolate theor each signal-transmitting section and the or each signal-receivingsection from the neighboring section or sections. This structure cansignificantly suppress the crosstalk caused by a current flowing throughthe Si substrate. Although, the Si substrate is separated toelectrically isolate the foregoing sections from each other, theseparated portions are mechanically connected by the insulatingsubstrate at the time the components are mounted. Therefore,high-precision mounting is possible by exploiting the advantageousfeatures of the Si substrate.

The communications module of the present invention can considerablyreduce the following electrical crosstalks:

-   -   (a) with a single-channel tranceiver, the crosstalk between the        signal-transmitting section and the signal-receiving section;    -   (b) with a multiple-channel transmitter or receiver, the        crosstalk between the channels; and    -   (c) with a multiple-channel tranceiver, the crosstalk not only        between the signal-transmitting section and the signal-receiving        section in each channel but also between the channels.        As a result, a high-performance and tiny optical communications        module can be produced. Additional production steps are the        bonding of the insulating substrate to the Si substrate and the        separation of the Si substrate. Therefore, the optical        communications module is suitable for mass production at low        cost.

In accordance with an aspect of the present invention, a method forproducing an optical communications module comprises the followingsteps:

-   -   (a) An Si wafer is produced on which a multitude of segments to        be used as substrate chips for optical communications are        provided. Each of the segments is provided with:        -   (a2) at least one V-shaped groove or at least one optical            waveguide, and        -   (a2) a plurality of metallized patterns formed by            evaporation, etching, or printing.    -   (b) The Si wafer is cut into individual chips to obtain Si        substrates for optical communications.    -   (c) A pair of a signal-transmitting section and a        signal-receiving section, a plurality of signal-transmitting        sections, a plurality of signal-receiving sections, or a        plurality of pairs of a signal-transmitting section and a        signal-receiving section are mounted onto one of the Si        substrates.    -   (d) An insulating substrate is bonded to the back face of the Si        substrate.    -   (e) A separating groove is provided to separates the Si        substrate along the or each boundary line between the foregoing        sections in order to prevent the electrical crosstalk through        the Si substrate. In this case, the separating groove is        provided from the top surface of the Si substrate to some        midpoint of the insulating substrate.

In accordance with another aspect of the present invention, a method forproducing an optical communications module comprises the followingsteps:

-   -   (a) An insulating substrate is bonded to the back face of an Si        wafer.    -   (b) A multitude of segments to be used as chips for optical        communications are provided on the Si wafer. Each of the        segments is provided with:        -   (b1) at least one V-shaped groove or at least one optical            waveguide;        -   (b2) a plurality of metallized patterns formed by            evaporation, etching, or printing;        -   (b3) a pair of a signal-transmitting section and a            signal-receiving section, a plurality of signal-transmitting            sections, a plurality of signal-receiving sections, or a            plurality of pairs of a signal-transmitting section and a            signal-receiving section; and        -   (b4) a separating groove that separates the Si wafer along            the or each boundary line between the foregoing sections in            order to prevent electrical crosstalk.

In this case, the separating groove is provided from the top surface ofthe Si wafer to some midpoint of the insulating substrate.

-   -   (c) The Si wafer is cut into individual chips.

In accordance with yet another aspect of the present invention, a methodfor producing an optical communications module comprises the followingsteps:

-   -   (a) An Si wafer is produced on which a multitude of segments to        be used as substrate chips for optical communications are        provided. Each of the segments is provided with:        -   (a1) at least one V-shaped groove or at least one optical            waveguide;        -   (a2) a plurality of metallized patterns formed by            evaporation, etching, or printing; and        -   (a3) a plurality of spaces each of which is to be used for            mounting a signal-transmitting section or a signal-receiving            section.    -   (b) The Si wafer is cut into individual chips to obtain Si        substrates for optical communications.    -   (c) An insulating substrate is bonded to the back face of one of        the Si substrates.    -   (d) A separating groove is provided that separates the Si.        substrate along the or each boundary line between the spaces in        order to prevent the electrical crosstalk through the Si        substrate. In this case, the separating groove is provided from        the top surface of the Si substrate to some midpoint of the        insulting substrate.    -   (e) A pair of a signal-transmitting section and a        signal-receiving section, a plurality of signal-transmitting        sections, a plurality of signal-receiving sections, or a        plurality of pairs of a signal-transmitting section and a        signal-receiving section are mounted in the spaces on the Si        substrate.

In accordance with yet another aspect of the present invention, a methodfor producing an optical communications module comprises the followingsteps:

-   -   (a) An insulating substrate is bonded to the back face of an Si        wafer.    -   (b) A multitude of segments to be used as chips for optical        communications are provided on the Si wafer. Each of the        segments is provided with:        -   (b1) at least one V-shaped groove or at least one optical            waveguide;        -   (b2) a plurality of metallized patterns formed by            evaporation, etching, or printing;        -   (b3) a plurality of spaces each of which is to be used for            mounting a signal-transmitting section or a signal-receiving            section; and        -   (b4) a separating groove that separates the Si wafer along            the boundary line between the spaces in order to prevent            electrical crosstalk.

In this case, the separating groove is provided from the top surface ofthe Si wafer to some midpoint of the insulating substrate.

-   -   (c) A pair of a signal-transmitting section and a        signal-receiving section, a plurality of signal-transmitting        sections, a plurality of signal-receiving sections or a        plurality of pairs of a signal-transmitting section and a        signal-receiving section are mounted in the spaces of each of        the segments.    -   (d) The Si wafer is cut into individual chips.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a plan view showing the optical communications module ofExample 1 of the present invention.

FIG. 2 is a lateral cross section showing the optical communicationsmodule of Example 1 of the present invention.

FIG. 3 is a plan view showing the optical communications module ofExample 2 of the present invention.

FIG. 4 is a longitudinal cross section showing the opticalcommunications module of Example 2 of the present invention.

FIG. 5 is a plan view showing one chip worth of Si wafer on whichV-shaped grooves and metallized patterns are provided by waferprocessing during the process. for producing the optical communicationsmodule of the present invention having a signal-transmitting section anda signal-receiving section.

FIG. 6 is a plan view showing the Si substrate chip shown in FIG. 5after being separated from the rest of the Si wafer and processed by thefollowing steps: (a) an insulating substrate is bonded to the back faceof the Si substrate, (b) lead pins are bonded to the insulatingsubstrate, and (c) the Si substrate is separated by a separating groove.

FIG. 7 is a plan view showing the Si substrate in which optical fibersare fixed in the V-shaped grooves, and an LD, a PD, and metallizedpatterns are connected to lead pins by wire bonding.

FIG. 8 is a plan view showing the Si substrate in which the spacesbetween one of the optical fibers and the LD and between the otheroptical fiber and the PD are filled with a transparent resin, theseparating groove is filled with a black resin, and the entire unit iscovered with an encapsulating resin.

FIG. 9 is a plan view of an optical tranceiver module of a prior art inwhich a signal-transmitting section (LD) and a signal-receiving section(PD) are placed with left-right symmetry on an Si substrate having twoparallel V-shaped grooves, and optical fibers are fitted in the V-shapedgrooves.

FIG. 10 is a longitudinal cross section of the optical tranceiver moduleshown in FIG. 9.

FIG. 11 is a lateral cross section at a plane including the LD and PD inthe optical tranceiver module shown in FIG. 9.

FIG. 12 is a perspective view showing the optical communications moduleof Example 3 of the present invention.

FIG. 13 is a perspective view showing the optical communications moduleof Example 4 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Desirable embodiments of the optical communications module of thepresent invention are explained below in detail by showing the principalcomponents and by referring to the accompanying drawings. To avoidduplicated explanations, the same signs are allocated to the samemembers throughout the drawings. The ratio of the dimensions in thedrawings do not necessarily coincide with the explanation.

[A. Si Substrate]

A single-crystalline Si substrate is used to effectively increase themounting accuracy. For this purpose, a (001) single-crystalline Sisubstrate is used because it is advantageous in accurately forming theV-shaped grooves in terms of the angle and breadth by anisotropicetching. The mounting positions of the LD and PD are indicated bymarking in advance. The thickness may be the same as or thinner thanthat of the Si substrate used for ordinary surface mounting.

[B. Optical Path]

An optical path on the Si substrate may be formed either by an opticalfiber or by an optical waveguide. An optical fiber is fitted in aV-shaped groove formed on the Si substrate and is fixed there.

[C. Insulating Substrate]

The Si substrate is supported by an insulating substrate. Without theinsulating substrate, the below-mentioned separating groove cannot beformed. The size of the insulating substrate may be the same as orslightly larger than that of the Si substrate. A ceramic or plasticsubstrate is used as the insulating substrate. The insulating substratemay be bonded either to the back face of an Si wafer before cutting intochips or to the back face of the Si substrate.

[D. Separating Groove]

A separating groove is formed to provide electrical isolation between asignal-transmitting section and a signal-receiving section, betweensignal-transmitting sections, and between signal-receiving sections. Theseparating groove has a depth exceeding the thickness of the Sisubstrate and penetrating into the insulating substrate to a certainextent. When the Si substrate carries one signal-transmitting sectionand one signal-receiving section, one separating groove is needed. Whenthe Si substrate carries more than two optical devices, multipleseparating grooves are needed.

[E. Combination of a Signal-Transmitting Section and a Signal-ReceivingSection]

When the Si substrate carries one signal-transmitting section and onesignal-receiving section, one separating groove is provided between thetwo sections. When the two sections are placed with left-right symmetry,the separating groove is provided longitudinally along the bisector.When one section is placed in the front and the other in the rear, theseparating groove is provided laterally between the two sections.

[F. Combination of a Plurality of Signal-Transmitting Sections]

When the Si substrate carries a plurality of signal-transmittingsections, a separating groove or separating grooves are provided alongthe individual boundary lines.

[G. Combination of a Plurality of Signal-Receiving Sections]

When the Si substrate carries a plurality of signal-receiving sections,a separating groove or separating grooves are provided along theindividual boundary lines.

[H. Combination of a plurality of Signal-Transmitting Sections andSignal-receiving sections]

When the Si substrate carries a plurality of signal-transmittingsections and signal-receiving sections, separating grooves are providedalong the individual boundary lines.

EXAMPLE 1 1. Two Optical Fibers, One Signal-Transmitting Section, andOne Signal-Receiving Section: FIGS. 1 and 2

FIGS. 1 and 2 show the optical communications module of Example 1. TheSi substrate 2 is divided into two sections. An insulating substrate 29is bonded to the back face of the rectangular Si substrate 2. Insulatinglayers 3 (SiO₂ layer) are formed on the rear half of the Si substrate 2to insulate the optical devices and wiring from the substrate.

The insulating layer 3 may be an SiO₂ layer obtained by thermallyoxidizing the Si substrate. Metallized patterns 20, 22, 23, and 24 areformed on the insulating layer 3. An LD 4 is bonded onto the metallizedpattern 20. The electrode on the LD 4 is connected to the metallizedpattern 22 with a wire 25. A PD 5 is bonded onto the metallized pattern24. The PD 5 is a side-illuminated type, which is also called awaveguide type. Light incident at the side face of the PD propagatesalong the waveguide and reaches the light-absorbing layer to generatephotoelectric current. The electrode on the PD 5 is connected to themetallized pattern 23 with a wire 26.

Parallel lead pins 34 to 37 protrude from the rear end of the insulatingsubstrate 29. They function as interfaces with the outside circuits. Themetallized pattern 20 is connected to the lead pin 34 with a wire 38,and the metallized pattern 22 to the lead pin 35 with a wire 39. Theseconnections supply driving currents to the LD 4. Similarly, themetallized pattern 23 is connected to the lead pin 36 with a wire 42,and the metallized pattern 24 to the lead pin 37 with a wire 43. Theseconnections draw out photoelectric currents from the PD 5.

Two parallel V-shaped grooves 7 and 8 are formed on the front half 6 ofthe Si substrate 2. Optical fibers 9 and 10 are fitted in the V-shapedgrooves 7 and 8 and fixed there with an adhesive. After this process, aseparating groove 30 is formed to separate the Si substrate 2 along thelongitudinal bisector between the signal-transmitting section 32 and thesignal-receiving section 33. Not-withstanding the above description, theseparating groove 30 may be formed before the mounting of thesignal-transmitting section 32 and the signal-receiving section 33.

As shown in FIG. 2, which is a lateral cross section, the Si substrate 2is sepaated at the center. The separating groove 30 penetrates into theinsulating substrate 29 to. a certain extent. The presence of theinsulating substrate 29 can maintain the solidity not only of the Sisubstrate 2 but also of the LD, PD, and optical fibers on the Sisubstrate. The presence of the separating groove 30 can electricallyisolate the signal-transmitting section 32 and the signal-receivingsection 33 from each other. A transparent resin 44 fills the spacebetween the optical fiber 9 and the LD 4. Another transparent resin 44fills the space between the optical fiber 10 and the PD 5. Thetransparent resins 44 are provided to reduce the reflection loss at theend faces of the optical fibers. Furthermore, a black resin 45 fills theseparating groove 30. The black resin 45 prevents the scattered light ofthe LD 4 at the signal-transmitting section from entering the PD 5 atthe signal-receiving section. In other words, the black resin 45prevents optical crosstalk. In addition, the entire unit is molded witha stiff encapsulating resin 46 such as epoxy resin to complete theproduction of the resin package-type optical tranceiver module.

As with the prior art explained by referring to FIGS. 9 to 11,resistances R4 and R5 actually exist at the signal-transmitting section32, and resistances R6 and R7 actually exist at the signal-receivingsection 33, because the Si substrate 2 is electrically conductive to acertain degree. Capacitances C2 and C3, also, exist in the insulatinglayer 3.

As distinct from the foregoing prior art, the present invention providesthe separating groove 30, which isolates the signal-transmitting section32 and the signal-receiving section 33 from each other on the Sisubstrate 2. The separating groove 30 produces an additional resistanceR8. The amount of R8 is nearly infinity and can be expressed as R8=∞.Consequently, the total summation of the resistances is infinity. On theother hand, the separating groove 30 also produces an additionalcapacitance C9. Therefore, the total impedance is expressed asZ=1/ωC2+1/jωC3+R4+R5+1/jωC9+R6+R7. However, the capacitance C9 isextremely small, making the total impedance extremely high. As a result,the electrical crosstalk between the signal-transmitting section and thesignal-receiving section becomes extremely small.

The two parallel optical fibers connected to the LD and PD in Example 1can be replaced with two parallel optical waveguides formed on the Sisubstrate 2. The optical waveguides are formed at the places providedfor the V-shaped grooves 7 and 8 in FIG. 1. The end faces of the twooptical fibers from the outside are bonded to the side face of the Sisubstrate 2. Because the structure is similar to that of Example 1, nofurther explanation is needed. In this case also, the separating groove30 is provided longitudinally to electrically isolate thesignal-transmitting section and the signal-receiving section from eachother so that the electrical crosstalk can be reduced.

EXAMPLE 2 2. Y-branched optical waveguide, one signal-transmittingsection, and one signal-receiving section: FIGS. 3 and 4

FIGS. 3 and 4 show the optical communications module of Example 2. Inthis example, the signal-transmitting section and the signal-receivingsection are placed in a front and rear configuration on the Sisubstrate, not in a left-right configuration. An insulating substrate 48is bonded to the back face of the rectangular Si substrate 47. Awave-guiding layer (SiO₂ layer) is formed on the front half and centralregion of the Si substrate 47. Optical waveguides 49, 50, and 52 areformed in the shape of the curved letter Y in the wave-guiding layer.The optical waveguides may be silica glass optical waveguides made ofGeO₂-doped SiO₂ enclosed by SiO₂ or organic optical waveguides made ofpolyimide fluoride. The rear region of the Si substrate 47 forms a stepslightly lower than the neighboring region. An LD 53 is mounted on thestep at the place where the LD 53 faces the end of the optical waveguide50. A front region 54, also, forms a step slightly lower than theneighboring region. A PD 55 is mounted on the step at the place wherethe PD 55 faces the end of the optical waveguide 49.

A WDM filter 59 is embedded at the junction point of the opticalwaveguides 49, 50, and 52. An optical fiber 57 is bonded to the end face56 of the optical waveguide 52. Incoming light (1.55 μm) from theoptical fiber 57 propagates along the optical waveguide 52, isselectively reflected at the WDM filter 59, enters the optical waveguide49, and enters the PD 55 to generate photoelectric current. Outgoinglight (1.3 μm) emitted from the LD 53 enters the optical waveguide 50,passes through the WDM filter 59, propagates along the optical waveguide52, and enters the optical fiber 57 to propagate to the outside.

As explained above, when the LD 53 and the PD 55 are placed in a frontand rear configuration, a separating groove 58 is provided between theLD and PD. Although the separating groove 58 separates the Si substrate47 completely, it penetrates into the insulating substrate 48 onlyslightly, leaving almost entire thickness of the insulating substrate 48intact. Therefore, the insulating substrate 48 can maintain the solidityof the entire unit as an integrated body. The separating groove 58 isadvantageous because it reduces the electrical crosstalk between thesignal-transmitting section and signal-receiving section.

3. Module Having Only a Plurality of Signal-Transmitting Sections

The present invention can also be applied to a complex transmittermodule having signal-transmitting sections LD1, LD2, . . . , and LDm onthe Si substrate. An insulating substrate is bonded to the back face ofthe Si substrate, and at least one separating groove is provided on theSi substrate along the individual boundary lines of thesignal-transmitting sections LD1, LD2, . . . , and LDm. The separatinggroove or grooves can reduce the electrical crosstalk between thesignal-transmitting sections.

4. Module Having Only a Plurality of Signal-Receiving Sections

The present invention can also be applied to a complex receiver modulehaving signal-receiving sections PD1, PD2, . . . , and PDm on the Sisubstrate. An insulating substrate is bonded to the back face of the Sisubstrate, and at least one separating groove is provided on the Sisubstrate along the individual boundary lines of the signal-receivingsections PD1, PD2, . . . , and PDm. The separating groove or grooves canreduce the electrical crosstalk between the signal-receiving sections.

5. Module Having a Plurality of Signal-Transmitting Sections andSignal-Receiving Sections

The present invention can also be applied to a complex transceivermodule having signal-transmitting sections LD1, LD2, . . . , and LDm andsignal-receiving sections PD1, PD2, . . . , and PDm on the Si substrate.An insulating substrate is bonded to the back face of the Si substrate,and a plurality of separating grooves are provided on the Si substratealong the individual boundary lines of the signal-transmitting sectionsLD1, LD2 , . . . , and LDm and the signal-receiving sections PD1, PD2, .. . , and PDm. The separating grooves can reduce the electricalcrosstalk between the signal-transmitting sections, between thesignal-receiving sections, and between the signal-transmitting sectionand the signal-receiving section.

6. Method for Producing a Tranceiver Module Having a Separating Groove(FIGS. 5 to 8)

The tranceiver module of the present invention is produced by thefollowing process:

-   -   (a) An insulating substrate is bonded to the back face of the Si        substrate.    -   (b) Optical devices for at least one signal-transmitting section        and at least one signal-receiving section are mounted on the Si        substrate.    -   (c) Before or after the mounting of the optical devices, a        separating groove is provided between the sections to        electrically isolate each section from the neighboring section        or sections.        As distinct from the production process for an ordinary optical        communications module, the foregoing process has additional        steps of (a) and (c) described above.

FIG. 5 shows the Si substrate 2. Actually, a multitude of the samerectangular segments are formed on a large Si wafer longitudinally andlaterally by wafer processing. FIG. 5 shows a chip as one of them. Aninsulating layer 3 is formed on the rear half of the chip. Metallizedpatterns 20, 22, 23, and 24 are formed on the insulating layer 3.V-shaped grooves 7 and 8 are formed on the front half 6 of the chip.

The V-shaped grooves 7 and 8 having specified angles of inclination canbe formed by anisotropic etching with high precision. The metallizedpatterns can be formed by first forming a metallic layer by theevaporation or CVD method and then removing unneeded portions byetching. They can also be formed by printing. A multitude of the samesegments as shown in FIG. 5 are formed at the same time. Individualsegments are severed along the cleavage plane to produce individualchips. Each of them is the Si substrate 2.

An insulating substrate 29 is bonded to the back face of the Sisubstrate 2. The insulating substrate may be bonded to the back face ofthe Si wafer before the formation of the foregoing segments. Theinsulating substrate is produced by using a material such as a ceramicor plastic material. As shown in FIG. 6, lead pins 34 to 37 are bondedto the insulating substrate 29. A separating groove 30 is formed alongthe longitudinal bisector of the Si substrate 2. The separating groove30 reaches the insulating substrate 29 and penetrates into it onlyslightly. As a result, the Si substrate 2 is separated into the leftsection and the right section. The two sections look as if they are twoislands on the insulating substrate 29. Although FIG. 6 shows theprocess in which the separating groove 30 is formed before the opticaldevices are mounted, the separating groove 30 may be formed after theoptical devices are mounted.

As shown in FIG. 7, an LD 4 is bonded onto the metallized pattern 20,and a PD 5 onto the metallized pattern 24. Connections betweenmetallized patterns and between each metallized pattern and thecorresponding lead pin are performed by wire-bonding.

As shown in FIG. 8, the spaces between the optical fiber 9 and the LD 4and between the optical fiber 10 and the PD 5 are filled with atransparent resin 44, because light propagates through these spaces. Thetransparent resin 44 prevents the light from scattering due toreflection and improves the optical coupling between the optical fiberand the optical device. Next, the separating groove 30 is filled with ablack resin 45. Thus, a dual-substrate structure of the insulatingsubstrate and Si substrate is formed. The structure is covered with astiff encapsulating resin 46 such as epoxy resin by transfer molding toform a package-type module. FIG. 8 is a plan view illustrating theconfiguration of the module viewed from above. Actually, the top surfaceof the module is covered with the encapsulating resin 46.

EXAMPLE 3 7. Tranceiver Module Having a Y-Branched Optical Waveguide:FIG. 12

As explained above, Example 1 is provided with a signal-transmittingsection and a signal-receiving section placed in a left-rightconfiguration, and Example 2 is provided with a signal-transmittingsection and a signal-receiving section placed in a front and rearconfiguration. Even when the two sections are placed in a front and rearconfiguration, the module is not necessarily limited to the system inwhich the incoming light is reflected by the WDM filter as explained inExample 2. FIG. 12 shows Example 3 in which a signal-transmittingsection (LD) and a signal-receiving section (PD) are placed in a frontand rear configuration.

An insulating substrate 62 is bonded to the back face of a slender Sisubstrate 60. A wave-guiding layer is formed over the Si substrate 60.Optical waveguides 64, 66, and 67 are formed in the shape of the letterY together with a junction 65 in the wave-guiding layer. In this case,the junction 65 itself has a wavelength-selecting capability. An LD 69is mounted in the rear portion of the Si substrate 60 such that it facesthe end 72 of the optical waveguide 66.

A PD 68 is mounted at the place adjacent to the end of the opticalwaveguide 67. Although not shown in FIG. 12, metallized patterns areprovided at the places for the LD and PD on the Si substrate. The end ofthe optical fiber from the outside (not shown in FIG. 12) is bonded tothe end 73 of the optical waveguide 64. The metallized patterns, LD, andPD are connected to lead pins with wires. The entire unit is molded witha resin. Because these steps are similar to those explained in theforegoing examples, detailed explanations are omitted.

Outgoing light (1.3 μm) emitted from the LD 69 enters the opticalwaveguide 66 at the end 72, passes through the junction 65, propagatesalong the optical waveguide 64, and enters the optical fiber (not shownin FIG. 12) at the end 73 to propagate to the outside.

Incoming light (1.55 μm) entering from the outside optical fiberpropagates along the optical waveguides 64 and 67, and enters the PD 68to generate photo-electric current. A separating groove 70 is providedbetween the end 72 of the optical waveguide 66 and the LD 69. Theseparating. groove 70 suppresses the electrical crosstalk between the LD69 and the PD 68.

EXAMPLE 4 8. Tranceiver Module: FIG. 13

As explained above, Example 1 is provided with a signal-transmittingsection and a signal-receiving section placed in a left-rightconfiguration, Example 2 is provided with a Y-shaped optical path, a WDMfilter, and a signal-transmitting section and a signal-receiving sectionplaced in a front and rear configuration, and Example 3 is provided witha Y-shaped optical path and a signal-transmitting section and asignal-receiving section placed in a front and rear configuration. FIG.13 shows Example 4 in which a signal-transmitting section and asignal-receiving section placed in a front and rear configuration areconnected by one optical waveguide.

An insulating substrate 75 is bonded to the back face of a slender Sisubstrate 74. A wave-guiding layer is formed over the Si substrate 74.One optical waveguide 76 is formed in the wave-guiding layer. Aseparating groove 77 separating the Si substrate 74 is provided at theend 79 of the optical waveguide 76. An LD 78 constituting thesignal-transmitting section is mounted behind the separating groove 77.Outgoing light from the LD 78 passes through the separating groove 77and enters the optical waveguide 76 at the end 79.

A WDM filter 82 is inserted and fixed at some midpoint in the opticalwaveguide 76 in an upwardly slanting position. A submount 83 having anoptical path within it is fixed immediately in front of the WDM filter82. A PD 84 constituting the signal-receiving section is mounted on thesubmount 83. The PD 84 is a rear-illuminated type. The separating groove77 can prevent the electrical crosstalk between the LD 78 and the PD 84.

1-7. (canceled)
 8. A method for producing an optical communicationsmodule, the method comprising the steps of: (a) producing an Si wafer onwhich a multitude of segments to be used as substrate chips for opticalcommunications are provided; each of the segments being provided with:(a1) one component selected from the group consisting of at least oneV-shaped groove and at least one optical waveguide; and (a2) a pluralityof metallized patterns formed by one method selected from the groupconsisting of evaporation, etching, and printing; (b) cutting the Siwafer into individual chips to obtain Si substrates for opticalcommunications; (c) mounting onto one of the Si substrates one memberselected from the group consisting of a pair of a signal-transmittingsection and a signal-receiving section, a plurality ofsignal-transmitting sections, a plurality of signal-receiving sections,and a plurality of pairs of a signal-transmitting section and asignal-receiving section; (d) bonding an insulating substrate to theback face of the Si substrate; and (e) providing a separating groovethat separates the Si substrate along the or each boundary line betweenthe sections constituting the selected member in order to prevent theelectrical crosstalk through the Si substrate, the separating groovebeing provided from the top surface of the Si substrate to some midpointof the insulating substrate.
 9. A method for producing an opticalcommunications module, the method comprising the steps of: (a) bondingan insulating substrate to the back face of an Si wafer; (b) providingon the Si wafer a multitude of segments to be used as chips for opticalcommunications; each of the segments being provided with: (b1) onecomponent selected from the group consisting of at least one V-shapedgroove and at least one optical waveguide; (b2) a plurality ofmetallized patterns formed by one method selected from the groupconsisting of evaporation, etching, and printing; (b3) one memberselected from the group consisting of a pair of a signal-transmittingsection and a signal-receiving section, a plurality ofsignal-transmitting sections, a plurality of signal-receiving sections,and a plurality of pairs of a signal-transmitting section and asignal-receiving section; and (b4) a separating groove that separatesthe Si wafer along the or each boundary line between the sectionsconstituting the selected member in order to prevent electricalcrosstalk, the separating groove being provided from the top surface ofthe Si wafer to some midpoint of the insulating substrate; and (c)cutting the Si wafer into individual chips.
 10. A method as defined byclaim 8, the method further comprising the steps of: (a) bonding aplurality of lead pins to the insulating substrate; (b) connecting eachlead pin to the corresponding signal-transmitting section,signal-receiving section, or metallized pattern; (c) fixing at least oneoptical fiber into the V-shaped groove or grooves; and (d) molding theentire unit with a resin.
 11. A method as defined by claim 9, the methodfurther comprising the steps of: (a) for performing the following steps,selecting one chip from the chips; (b) bonding a plurality of lead pinsto the insulating substrate of the chip; (c) connecting each lead pin tothe corresponding signal-transmitting section, signal-receiving section,or metallized pattern; (d) fixing at least one optical fiber into theV-shaped groove or grooves; and (e) molding the entire unit with aresin.
 12. A method for producing an optical communications module, themethod comprising the steps of: (a) producing an Si wafer on which amultitude of segments to be used as substrate chips for opticalcommunications are provided; each of the segments being provided with:(a1) one component selected from the group consisting of at least oneV-shaped groove and at least one optical waveguide; (a2) a plurality ofmetallized patterns formed by one method selected from the groupconsisting of evaporation, etching, and printing; and (a3) a pluralityof spaces each of which is to be used for mounting one section selectedfrom the group consisting of a signal-transmitting section and asignal-receiving section; (b) cutting the Si wafer into individual chipsto obtain Si substrates for optical communications; (c) bonding aninsulating substrate to the back face of one of the Si substrates; (d)providing a separating groove that separates the Si substrate along theor each boundary line between the spaces in order to prevent theelectrical crosstalk through the Si substrate, the separating groovebeing provided from the top surface of the Si substrate to some midpointof the insulating substrate; and (e) mounting in the spaces on the Sisubstrate one member selected from the group consisting of a pair of asignal-transmitting section and a signal-receiving section, a pluralityof signal-transmitting sections, a plurality of signal-receivingsections, and a plurality of pairs of a signal-transmitting section anda signal-receiving section.
 13. A method for producing an opticalcommunications module, the method comprising the steps of: (a) bondingan insulating substrate to the back face of an Si wafer; (b) providingon the Si wafer a multitude of segments to be used as chips for opticalcommunications; each of the segments being provided with: (b1) onecomponent selected from the group consisting of at least one V-shapedgroove and at least one optical waveguide; (b2) a plurality ofmetallized patterns formed by one method selected from the groupconsisting of evaporation, etching, and printing; (b3) a plurality ofspaces each of which is to be used for mounting one section selectedfrom the group consisting of a signal-transmitting section and asignal-receiving section; and (b4) a separating groove that separatesthe Si wafer along the boundary line between the spaces in order toprevent electrical crosstalk, the separating groove being provided fromthe top surface of the Si wafer to some midpoint of the insulatingsubstrate; (c) mounting in the spaces of each of the segments one memberselected from the group consisting of a pair of a signal-transmittingsection and a signal-receiving section, a plurality ofsignal-transmitting sections, a plurality of signal-receiving sections,and a plurality of pairs of a signal-transmitting section and asignal-receiving section; and (d) cutting the Si wafer into individualchips.
 14. A method as defined by claim 12, the method furthercomprising the steps of: (a) bonding a plurality of lead pins to theinsulating substrate; (b) connecting each lead pin to the correspondingsignal-transmitting section, signal-receiving section, or metallizedpattern; (c) fixing at least one optical fiber into the V-shaped grooveor grooves; and (d) molding the entire unit with a resin.
 15. A methodas defined by claim 13, the method further comprising the steps of: (a)for performing the following steps, selecting one chip from the chips;(b) bonding a plurality of lead pins to the insulating substrate of thechip; (c) connecting each lead pin to the correspondingsignal-transmitting section, signal-receiving section, or metallizedpattern; (d) fixing at least one optical fiber into the V-shaped grooveor grooves; and (e) molding the entire unit with a resin.